1. Field of the Invention
The present invention relates to an ink supply controller for controlling the amount of ink supply in a printing apparatus including an ink supply means such as an ink fountain device.
2. Description of the Background Art
A typical offset printing apparatus includes an ink supply device or the like having a plurality of ink keys to variably adjust the amount of ink supply to each of a plurality of regions (ink key regions) extending in the feed direction of a printing sheet. This ink supply device controls the amount of ink supply in accordance with an image area percentage on a printing plate. An example of such a technique, as disclosed in Japanese Patent Application Laid-Open No. 11-268394 (1999), is as follows. Data (ink curve data) indicating a relationship between the image area percentage and the amount of ink supply (or an ink key opening) are previously stored in a database or the like. The image area percentage of an image is measured for each of the ink key regions, and the ink key opening is adjusted in accordance with each measurement result so that an optimum printing density is achieved.
However, the relationship between the image area percentage and the ink key opening varies depending on printing conditions susceptible to a printing environment. For this reason, the above-mentioned database or the like must be updated for each print job.
One of the simplest methods of updating the database is considered to include updating the database each time a new print job is executed. However, if the new print job is to print an image such that the values of the image area percentage are distributed locally within some limited range, resultant ink curve data is of low accuracy outside this range.
FIG. 6 shows an example of distribution of the ink key opening versus the image area percentage by using dots D1 to D6 in the form of solid circles when the values of the image area percentage in the respective ink key regions are distributed locally within some limited range in the above-mentioned manner, that is, when there is a small difference in image area percentage between the ink key regions. This example corresponds to an instance in which there is no data in a high image area percentage region since the overall low color density concerned on a print image of a color corresponding to the printing plate results in low image area percentage. In such a case, if the ink curve data is updated using only the data obtained when executing the new print job, there is a likelihood that error or deviation from the proper ink curve data is increased in the high image area percentage region.
The present invention is intended for a technique for controlling supply of ink in a printing apparatus.
According to the present invention, a printing apparatus having a plurality of ink keys for controlling the supply of ink while adjusting the opening of each of the ink keys in accordance with predetermined ink curve data to thereby perform printing, comprises: a data storing element for storing at least one first ink curve data; a data acquiring element for acquiring a plurality of data values each indicating a corresponding relationship between an image area percentage of a printing plate and a convergent value of the opening of each of the ink keys; and an ink curve generating element for generating second ink curve data based on one first ink curve data selected from the at least one first ink curve data and the plurality of data values.
Preferably, the ink curve generating element acquires a first parameter and a second parameter to generate the second ink curve data from the first and second parameters, the first parameter being at least one data value indicative of the one first ink curve data, the second parameter being at least one of the plurality of data values.
Preferably, the ink curve generating element generates the second ink curve data from an approximate curve obtained from a distribution of the first and second parameters represented in a two-dimensional coordinate system.
For execution of a new print job, the ink curve is generated by using not only the data about the distribution of the convergent key opening versus the image area percentage in each ink key region for a printing plate to be used but also the data about the ink curve used in previous printing. This allows the generation of a proper ink curve even when a printing plate for the new print job has a small difference in image area percentage between the ink key regions.
It is therefore an object of the present invention to provide a printing apparatus capable of generating a proper ink curve from a result of printing even when there is a small difference in image area percentage between ink key regions.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.